Triads are chords that contain only three notes. Chords such as C major, F major etc are triads. These types of chords are often referred to just by their letter name, i.e., C or F, so just "C" or C major are both the same chord and they are also both triads.
Minor chords like A minor, E minor etc are also triads.
Dominant chords such as seventh chords and ninth chords all contain more than three notes so these are not classed as triads. An easy rule to remember is if a chord name doesn't contain a number then it's normally a triad with the exception of sus4, sus2. The indication of a number doesn't guarantee that a chord is not a triad but most of the time numberless chords do refer to it being triadic.
Incidentally, power chords which are typically called fifths are neither triads nor chords, they are just intervals, although most guitarists will call them chords. More detailed information about triads can be found at Wikipedia.